1. Field of the Invention
The present invention relates to a conductive polymer and a solid electrolytic capacitor using the same.
2. Description of the Related Art
As electronic equipments become smaller, faster, and digitized, a low-impedance capacitor having small size, large capacity, and good high-frequency properties is also strongly demanded for the capacitor used.
Capacitors used in the high frequency region have been conventionally mainly multilayer ceramic capacitors, but it is difficult to address the needs of small size, large capacity, and low impedance with the multilayer ceramic capacitors.
As large-capacity capacitors, aluminum electrolytic capacitors, tantalum solid electrolytic capacitors, and the like have been conventionally used, but electrolytic solutions or electrolytes used for these, for example, manganese dioxide, have high resistance, so that it is difficult to achieve low impedance in the high frequency region.
In order to achieve low impedance in the high frequency region for these, a solid electrolytic capacitor using a conductive polymer is proposed. In such a capacitor, the resistance of the conductive polymer is lower than that of the electrolyte and manganese dioxide, so that the capacitor has low impedance.
In recent years, a demand for further lower impedance has been strong, and the higher conductivity of the conductive polymer used for the capacitor has been required. FIG. 1 is a schematic diagram of the internal structure of a solid electrolytic capacitor. In such a solid electrolytic capacitor, in many cases, an anode oxide film is formed on an anode body 1 having a porous shape to provide a dielectric layer 2, further, a solid electrolyte layer 3 is formed on the dielectric layer 2, and a graphite layer 4 and a silver layer 5 are formed in this order to provide a cathode, as shown in FIG. 1.
As the conductive polymer used as the solid electrolyte of such a solid electrolytic capacitor, polyaniline, polypyrrole, and poly(3,4-ethylenedioxythiophene) are generally known. In order to optimize the use of such existing compounds, the examination of additives and the like is promoted. For example, JP 3241636 B discloses a method for manufacturing a solid electrolytic capacitor in which a resin as a binder is added to a solution of the monomer of a conductive polymer in the step of forming a conductive polymer layer in the manufacture of a solid electrolytic capacitor.
On the other hand, attempts to achieve higher conductivity by controlling the skeleton, rather than by external factors such as additives, are also reported. For example, JP 01-011209 B discloses a technique for poly(3-alkylthiophene). According to this document, it is alleged that poly(3-alkylthiophene) can be easily manufactured by electrolytic polymerization of 3-alkylthiophene as a monomer in a solvent containing a supporting electrolyte. Further, it is alleged that the obtained poly(3-alkylthiophene) exhibits a conductivity of 60 S/cm to 95 S/cm.
JP 02-283722 A discloses a method for manufacturing a conductive polymer with 3-alkylthiophene as a monomer. According to this document, it is alleged that poly(3-alkylthiophene) having high conductivity can be manufactured by polymerizing 3-alkylthiophene as a monomer in a reaction medium containing ferric salt, alkyl halide, and water. For example, in JP 05-222129 A, it is alleged that when the polymer, which is obtained by removing low molecular weight poly(3-alkylthiophene), has a number average molecular weight of 15,000 or more containing 10% or less of polymer having a molecular weight of 10,000 or less, the conductivity is dramatically improved.
For example, T-A Chen and R. D. Rieke, Synthetic Metals, 60 (1993), 175-177, reports that poly(3-alkylthiophene) highly sterically controlled with 3-alkylthiophene as a monomer has a conductivity of 1350 S/cm, and the conductivity is improved with respect to 5 S/cm for a polymer in which the monomer is randomly bonded. JP 2007-501300 A discloses a technique for a method for manufacturing such highly sterically controlled poly(3-alkylthiophene).
However, with the attempts to improve the conductivity by adding additives to existing conductive polymers as in conventional techniques, a certain effect is obtained, but the effect of dramatically improving the conductivity is not obtained.
When the monocyclic compounds as described in JP 01-011209 B, JP 02-283722 A and JP 05-222129 A are used as monomers, the obtained polymers all have random configurations, so that a dramatic improvement in conductivity cannot be expected.
With the methods for forming an undoped conductive polymer by coupling reaction as described in T-A Chen and R. D. Rieke, Synthetic Metals, 60 (1993), 175-177, and JP 2007-501300 A, the control of the doping ratio is difficult. Generally, there is an appropriate value for the doping ratio, and if the doping ratio is too high or too low, the conductivity decreases. Therefore, when such methods are industrially used, equipment and analysis for precise control of chemical reaction are necessary, which are expensive.
Further, the generally used anode body for the solid electrolytic capacitor is a porous body, so that penetration into the pores is necessary. With such related art in which a polymer solution is obtained, application to the solid electrolytic capacitor is limited. In other words, the polymer solution cannot be used as a solid electrolyte in the pores and is limited to use such as for external coating.
However, for such a polymer obtained as the polymer solution, polyalkylthiophene is known as a soluble polymer, but by polyalkylthiophene being sterically controlled, its crystallinity increases, so that its solubility in a solvent decreases remarkably. In such a case, for use such as for external coating, a certain thickness is demanded for the polymer film after drying, but the solubility is too low, so that a thick film cannot be formed. Therefore, application to the solid electrolytic capacitor is difficult. The object of the present invention is to provide a conductive polymer having high conductivity that can be manufactured at low cost, and a solid electrolytic capacitor having low ESR using the same.